There are three possible problems you could experience:

1. Popup Blocker
• Please allow popups for our online store:
• Chrome Desktop: How To
• Edge Desktop: How To
• Safari on Mac: How To
• Chrome Android: How To
• Safari iOS: How To
2. Second Factor Authorization
• The magic word behind the issue you've doesn't exist, it depends on the credit card you've:
• VISA: Verified by VISA
• MasterCard: 3D-Secure / Identity Check
• AMEX: SafeKey
• If you buy in Europe with your Credit Card, you've to verify your identity with a second code (first step: Credit Card number + CVV code from the backside). This code will be generated, e.g. in an app of your credit card company or an SMS from your credit card company, for every online purchase.
3. International Payment
• Our Payment Processing Provider is in Luxembourg, Europe. Please verify that your card can make international online payments in Europe, especially in Luxembourg.

I hope now you understand what you need or how your credit card company should be able to help you.